Our species is no longer influenced by climatic variations: it was in fact able to artificially create, within its own homes, a temperature more or less constant throughout the year, so as to guarantee the baby the same probability of survival in every season. The animals instead go into heat, and mate, only in the particular periods in which the female can be fertilized.

The frequency with which heat manifests itself, and its duration, vary in different species, and the determining factor is precisely the climate: the animals that live in places with constant climatic conditions all year round, elephants and giraffes for example, do not have certain periods in which to have inspiration.

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The females of animals that live in areas with distinct seasons, on the other hand, go into heat only in particular periods. With this stratagem they manage to give birth when the climatic conditions and the availability of food are the most suitable for the survival of the child. Some animals go into heat once a year, like some bats, bears, deer; others, like rabbits, repeatedly.